I. Technical Field
The present invention relates to a method of adhesively forming a solid thin film on the surface of a parent material.
II. Description of the Related Art
In recent years, functional thin films such as a solid lubricating film and a bioactive film have been under active research. Of the functional thin films, the solid lubricating film including a diamond-like carbon (DLC) film exhibits excellent tribology (friction, wear, and lubrication) characteristics and is thus brought to attention in various fields.
However, the adhesion of the solid lubricating film to the parent material is not sufficient, with the result that a critical problem such as peeling occurs in a high-surface-pressure use. Therefore, it is difficult to say that applications to a tool or a part whose contact surface pressure is high have been advanced well.
When the surface of an implant for a femur or a tooth is coated with a hydroxyapatite film, the bioactivity of osteoblasts on the surface of the implant is improved, so that a preferable bonding state with the bone is obtained. However, in some cases, peeling between the hydroxyapatite film and the main body of the implant becomes a large problem after an operation. Therefore, it is desirable to improve the adhesion of the coating film.
In order to solve the problems, research has been conducted for forming ripples on the surface of the parent material by grinding or sandblasting to improve the adhesion of the coating film. The ripples formed by grinding or sandblasting have random shapes because of the limitation of processing. A processing scale on the surface remains at the order of several μm to several tens of μm. In this case, the improvement of the adhesion of the coating film requires the formation of the ripples equal to or larger than the thickness of the coating film. Therefore, there is a report in which abrasive wear occurs or an effect cannot be obtained depending on a test method (Akio Motoi, Seiji Kataoka, and Kazuo Morikawa: Evaluation of adhesion of DLC film on mold for plastic forming, Bulletin of Tokyo Metropolitan Industrial Technology Research Institute, 6 (2003)).
In contrast, a patent application discloses that a nanostructure of approximately 1/10 of a wavelength order of a femtosecond laser beam can be formed using the femtosecond laser beam (JP 2003-211400 A). The nanostructure is formed in a long and thin fiber shape orthogonal to a polarization direction of the laser beam. For example, when a DLC film is irradiated with 300 shots at a fluence of 0.15 J/cm2, a periodic structure in which an average interval is 100 nm and a length is 200 nm to 2000 nm is obtained. The periodic structure has a pitch which significantly depends on the fluence (N. Yasumaru, K. Miyazaki, and J. Kiuchi: Femtosecond-laser-induced nanostructure formed on hard thin films of TiN and DLC, Appl. Phys. A 76 (2003) 983). Therefore, when a laser beam of a normal Gaussian mode is emitted for scanning, there is a problem in that a periodic structure having protrusion portions and groove portions which are provided with high continuity at regular periodic pitches cannot be formed in a wide area because a periodic pitch is changed between a center portion and a peripheral portion. The pitch of the nanostructure also depends on the number of irradiation of the laser beam, so the periodic pitch changes in an overlapped portion and the continuity of the periodic structure is deteriorated. This phenomenon is specific to the femtosecond laser beam, so a nanosecond laser beam or a picosecond laser beam cannot be used. The nanostructure has a size equal to or smaller than a wavelength of light, so the nanostructure cannot be monitored using an optical microscope.
On the other hand, it has been known that a grating periodic structure is formed by interference of incident light and surface scattered light when a polymer is irradiated with a linearly polarized laser beam at a fluence close to a processing threshold, (see, for example, P. E. Dyer and R. J. Farley: Periodic surface structure in the excimer laser ablative etching polymers, Appl. Phys. Lett., 57, 8 (1990) P.765, H. Hiraoka and M. Sendova: Laser-induced sub-half-micrometer periodic structure on polymer surfaces, App. Phys. Lett., 64, 5 (1994) P.563, and M. Bolle and S. Lazare: Submicron periodic structures produced on polymer surfaces with polarized excimer laser ultraviolet radiation, Appl. Phys. Lett., 60, 6 (1992) P.674). It is reported that the same structure can be formed for a metal or a semiconductor and the periodic pitch thereof is changed according to an irradiation angle (see, for example, A. E. Siegman, P. M. Fauchet: Stimulated Wood's anomalies on laser-illuminated surfaces, IEEE J. Quantum Electron, QE-20, 8 (1986) P.1384 and Yukimasa Minami and Koichi Toyoda: Incident-angle dependency of laser-induced surface ripples on metals and semiconductors, Review of Laser Engineering, 28, 12 (2000) P.824).
In any case, the periodic structure of the wavelength order of the laser beam is formed. A formation area of the periodic structure is limited to a spot area of the laser beam, so an application range is limited to an extremely narrow range. However, the inventors of the present invention have recently found that the grating periodic structure of the wavelength order can be formed in a wide area when scanning using the linearly polarized laser beam at the fluence close to the processing threshold is performed with an overlap (Hiroshi Sawada, Kousuke Kawahara, Takafumi Ninomiya, Kou Kurosawa, and Atsushi Yokotani: Formation of precise periodic structures using femtosecond laser, Journal of the Japan Society for Precision Engineering, 69, 4 (2003) 554). When such a processing method is used, the grating periodic structure having the protrusion portions and the groove portions which are provided with high continuity at regular periodic pitches equal to the wavelength order can be extremely easily formed in principle. In addition, the orientation of the periodic structure can be arbitrarily set by only adjusting the polarization direction. When this method is used for a disk-shaped test piece which is rotatable, various periodic structure patterns such as a radial pattern, a concentric pattern, and a spiral pattern can be formed. The periodic structure has a rainbow appearance because of a light diffraction phenomenon, so the presence or absence of the periodic structure can be easily visually checked.
The grating periodic structure has the protrusion portions and the groove portions which are uniformly provided with high continuity. Therefore, an effect for dramatically reducing friction and wear which are caused by the generation of a fluid pressure, a function for preventing the jamming of wear powders caused by the discharge of the wear powders through the groove portions, an effect for reducing adhesion, an effect for increasing a fatigue strength, and the like are recognized in sliding tests carried out by the inventors of the present invention in this application (Hiroshi Sawada, Kousuke Kawahara, Takafumi Ninomiya, Atsunobu Mori, and Kou Kurosawa Effect of precise periodic structures with femtosecond-laser on tribological characteristics under sliding tests, Journal of the Japan Society for Precision Engineering, 70, 1 (2004) 113 and Takeshi Furuno, Atsunobu Mori, Norio Tagawa, Hiroshi Sawada: Effect of precise periodic structures on fatigue resistance of rolling-sliding contact surface, Proceedings of JAST Tribology Conference, Tokyo, 2004-5, (2004) 119).
It is determined that the grating periodic structure has surface functions such as cell sensitivity and a wettability control function.
The grating periodic structure can be formed using a nanosecond laser beam or a picosecond laser beam. When a femtosecond laser beam whose pulse width is short is used, a thermal influence is small, so that processing can be performed with fewer disturbances. Therefore, it is suitable to use the femtosecond laser beam.    Non-Patent Document 1    Akio Motoi, Seiji Kataoka, and Kazuo Morikawa: Evaluation of adhesion of DLC film on mold for plastic forming, Bulletin of Tokyo Metropolitan Industrial Technology Research Institute, 6 (2003).